Systems and process for forming carbon nanotube sensors

ABSTRACT

A process for forming a functionalized sensor for sensing a molecule of interest includes providing at least one single or multi-wall carbon nanotube having a first and a second electrode in contact therewith on a substrate; providing a third electrode including a decorating material on the substrate a predetermined distance from the at least one single or multi-wall carbon nanotube having a first and a second electrode in contact therewith, wherein the decorating material has a bonding affinity for a bioreceptors that react with the molecule of interest; and applying a voltage to the third electrode, causing the decorating material to form nanoparticles of the decorating material on the at least one single or multi-wall carbon nanotube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the use of carbon nanotubetechnology in the field of sensor applications. More specifically, thisinvention relates to processes for forming and functionalizing of carbonnanotube field effect transistors (“CNTFETs”) for use in specific sensorsystems, such as chemical and biological sensors.

2. Description of the Related Art

Sensor technology has long been an active area of interest to numerousentities including the medical community, law enforcement, nationaldefense and basic research. The ability to accurately detect and analyzethe presence (or absence) of various molecules in central to manyapplications. By way of specific example, the medical communitycontinues to seek non-invasive or minimally invasive ways to monitorpatient health. Commonly owned U.S. Pat. No. 6,887,202 describes variousapproaches to transdermal monitoring. The teachings and descriptions ofU.S. Pat. No. 6,887,202 are incorporated herein by reference in theirentirety.

The present application describes an improved sensor configurationcompatible with many of the configurations described in U.S. Pat. No.6,887,202, wherein the improved sensor meets current demands for reducedsize and improved sensing characteristics.

BRIEF SUMMARY OF THE INVENTION

A first embodiment includes a process for forming a functionalizedsensor for sensing a molecule of interest. The process includes:providing at least one single-wall carbon nanotube having a first and asecond electrode in contact therewith on a substrate; providing a thirdelectrode including a decorating material on the substrate apredetermined distance from the at least one single-wall carbon nanotubehaving a first and a second electrode in contact therewith, wherein thedecorating material has a bonding affinity for a bioreceptors that reactwith the molecule of interest; and applying a voltage to the thirdelectrode, causing the decorating material to form nanoparticles of thedecorating material on the at least one single-walled carbon nanotube.

A second embodiment describes a system for forming a functionalizedsensor for sensing a molecule of interest including: at least one carbonnanotube having a first and a second electrode in contact therewith on asubstrate; a third electrode including a decorating material on thesubstrate a predetermined distance from the at least one carbon nanotubehaving a first and a second electrode in contact therewith, wherein thedecorating material has a bonding affinity for a bioreceptors that reactwith the molecule of interest; wherein a voltage is applied to the thirdelectrode, causing the decorating material to form nanoparticles of thedecorating material on the at least one carbon nanotube.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a carbon nanotube field effect transistor (CNTFET)for use with embodiments of the present invention.

FIGS. 2( a)-2(b) illustrate representative and actual set-upconfigurations for functionalizing carbon nanotubes.

FIGS. 3( a)-3(d) illustrate a series of SEM images showing depositionparticulars at different voltages.

FIG. 4 illustrates is a schematic showing anchoring of receptors tonanoparticles deposited on a carbon nanotube.

FIG. 5 illustrates a schematic of a CNTFET which has been passivatedusing a self assembled monolayer (SAM) to avoid non-specific binding ofbioreceptors.

FIG. 6 illustrates intermolecular linking of SAM molecules.

FIG. 7 illustrates an alternative arrangement of the CNT and iSE.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an embodiments described herein, carbon nanotube field effecttransistors (“CNTFETs”) may be decorated with nanoparticles as furtherdescribed to facilitate the binding of chemical/biological molecules ofinterest thereto. CNTFETs generally includes one or more CNTs, one ormore electrodes contacting the two ends of the nanotube(s), aninsulating dielectric layer (e.g., SiO₂) on top of or underneath thenanotube(s), and a conductive gate (e.g., doped silicon (if underneathof the nanotube), or a metallic top-gate (if on top of the nanotube)within a few hundred nanometers to the nanotube but insulated by thedielectric layer. Various techniques are used to developed CNTsincluding discharge, laser ablation and chemical vapor deposition(“CVD”) and such techniques are well known to those skilled in the art.The electrodes may be patterned using photolithography or electrode beamlithography. Further, the electrodes may be metallic or non-metallic(e.g., conductive polymers, indium-tin-oxide (ITO) and the like.

By way of specific example, a single wall CNT (“SWCNT”) is used as theconduction channel in a CNTFET implemented as the sensor in the devicesdescribed herein. FIG. 1 shows a CNTFET including SWCNT, first andsecond electrodes (S=source; D=drain), insulating dielectric layer(e.g., SiO₂) and conductive gate (Si). One skilled in the art recognizesthat CNTFETs may be formed using multiple CNTs, including CNT films ornetworks, including multiple CNTs, as described in the teachings of A.Star, E. Tu, J. Niemann, J-C. P. Gabriel, C. S. Joiner, and C. Valcke,Proc Natl Acad Sci USA. 2006 Jan. 24; 103(4): 921-926 and T. Ozel, A.Gaur, J. A. Rogers, and M. Shim, Nano Lett., 2005, 5 (5), pp 905-911which are incorporated herein by reference. In order to prepare theSWCNT for use in particular sensing applications and integrated devices,a functionalization process is employed. Functionalizing the CNTrequires not only successful attachment of molecules to the CNT, butalso preservation of the CNT's intrinsic electronic properties. Inaddition, in a device environment, the specificity of the attachmentbecomes important since any unintended attachment will potentiallyintroduce noise to the device. While the present embodiments contemplatea conductive substrate and thus a constraint on types of materials thatmay be used, embodiments are contemplated which utilize a top gate andthus the substrate is not constrained and can include glass, ceramic,plastic, etc. Further, while the specific embodiment described hereinrefers to at least one single wall CNT, the use of multi wall CNTs arealso contemplated.

In a preferred embodiment, the SWCNT is functionalized usingelectrically controllable Au nanoparticle decoration. Referring to FIG.2( a), the set up for facilitating Au decoration includes the additionof a third electrode in close proximity to, but not directly in contactwith the SWCNT. Specifically, the third electrode serves as a source ofAu and is sacrificed after the Au deposition. The third electrode may bereferred to as the “in-situ sacrificial electrode” (iSE). In aparticular example, the iSE is composed of a bilayer of metals Cr/Au (Crused as an adhesion layer for Au—could also use other adhesion layers,e.g. Ti, Titanium), at approximately 25 Å and 800 Å, respectively, andis located approximately 5-20 μm from the contact electrodes and isapproximately 5 μm wide. Alternatively, the iSE could be a single layermetal that does no require a binder layer. FIG. 2( b) is an actual viewof the electrodeposition set-up. In operation, a drop of electrolyte PBSis applied to the device, and a positive voltage V_(d) is applied to thesacrificial electrode, with both contact electrodes grounded. Au atomsfrom the sacrificial electrode are oxidized by the positive potentialand dissolve in the PBS as Au ions, and are reduced at the groundedelectrode/CNT and redeposit as metal atoms. Here the applied voltage isreduced to avoid electrolysis of water, and finer adjustments will bemade later to control the size and density of Au nanoparticles.

By way of example, referring to FIGS. 3( a)-(d), the high sensitivity ofparticle density and size in accordance with changes in applied voltageis illustrated. The SEM images show a series of carbon nanotubesdecorated with Au nanoparticles. With the deposition time held constantat 2 minutes, by slightly increasing the deposition voltage V_(d), thenumber of nanoparticles increases substantially, and the size of thenanoparticles also shows an increasing trend, from 20 nm to as large as300 nm. Note that at all voltages, the sizes of the nanoparticles arenot homogenous, and the variation of particle size increases withdeposition voltage. At 1.3V, for this specific device, there is only onenanoparticle deposited on the nanotube, and the size of the nanoparticleis about 60 nm (FIG. 3( a)). This is of special interest for sensorapplications because it makes single molecule sensitivity possible byminimizing possible defects and noise caused by the nanoparticles. At1.32V, there are 4 nanoparticles, and the sizes are 170 nm, 160 nm, 100nm, 110 nm, respectively (FIG. 3( b)). At 1.34V, there are 7nanoparticles, and the diameters are 67 nm, 324 nm, 337 nm, 212 nm, 246nm, 201 nm and 33 nm, respectively (FIG. 3( c)). At the highest voltage1.36V, there are over 30 nanoparticles on the nanotube, and the sizeranges from 27 nm to over 300 nm (FIG. 3( d)). In the middle, there areover 10 nanoparticles with diameters less than 100 nm, and on the twosides, nanoparticles are typically much larger, with diameters ofhundreds of nanometers.

In alternative embodiments, other materials may be used as the depositmaterial, such as Ag. Appropriate changes to the deposition set up areimplemented to account of use of other deposit materials.

Once decorated with appropriate nanoparticles, select receptors can beanchored to the nanoparticles in order to prepare the CNTFET for use asa sensor. This is shown schematically in FIG. 4. Certain metals havestrong affinity to specific chemical groups, and the affinity can beutilized to realize effective surface modifications. For example, goldatoms are known to interact strongly with sulfur atoms in thiols andform a strong covalent bond. In the present embodiment, the Aunanoparticles are deposited on the CNT sidewalls as anchoring sites toimmobilize thiol-terminated bio-molecules to the nanotube for sensorapplications. The strong covalent bond between the gold and thiolprovides for a more robust bond compared to nonspecific adsorption ofbiomolecules onto the CNT sidewalls. In addition, the catalytic natureand suitability for binding to the thiol as well as the excellentconductivity of the metallic Au nanoparticles makes the delivery of thechemical event at the biomolecule to the CNT channel much easier.

In order to better control binding of the bioreceptors to the Aunanoparticles in particular and avoid non-specific binding to, forexample, the SiO₂ surface, a self assembled monolayer (SAM) may be used.An SAM is an organized layer of molecules which consists of a head and atail, with the head showing a specific affinity for a substrate, and thetail having a desired functional group at the terminal. SAMs have beenwidely used for surface property modifications in electronic devices,especially microelectrochemical systems (MEMS) and nanoelectromechanicalsystems (NEMS). Its working mechanism is shown in FIG. 5. The originalsubstrate has an affinity for head group of the SAM, and aftermodification, the substrate has the property of the terminal functionalgroup R. The use of an SAM provides for an effective method to combinethe desirable properties of the substrate such as electricalconductivity, mechanical robustness, to the SAM molecule's chemicalproperties.

In order to passivate the SiO₂ surface for nonspecific proteinadsorption, two properties are desired for the SAM molecules. First, thehead group should have a strong affinity to SiO₂ surface. Second, thetail should have good protein resistivity. Polyethylene glycol (PEG)SAMs including methoxy-terminated PEG 2000 have been shown to have goodprotein resistivity. To realize strong affinity to SiO₂, the silanefunctional group having the following molecular structure is added toone end of the molecule:

In the presence of water molecules, the silane functional groups of thePEG hydrolyze and form trisilanols. The trisilanols interact with theSiO₂ and hydrogen bond with the surface bound water molecules. With mildheating, the water molecule is lost and a covalent siloxane bond isformed. When the distance and orientation between the groups becomefavorable, the tri-silanol head groups can interact with each other andintermolecular crosslinking takes place as shown in FIG. 6. Thispassivation treatment helps to reduce any non-specific binding of thebioreceptors.

In a particular application of the devices and processes describedherein, the electrodeposited Au nanoparticles serve as specific bindingsites for thiol-terminated glucose oxidase, and the presence of glucosecan be detected using the redox reaction between glucose and glucoseoxidase. The particular electrical configurations and processes forsensing reactions within the CNTFETs are well known to those skilled inthe art. By way of example the teachings of A. Star et al. (cited above)and K. J. Cash, H. A Clark, Trends in Molecular Medicine, Volume 16,Issue 12, 584-593, 23 Sep. 2010 are hereby incorporate by reference intheir entirety. The glucose oxidase is functionalized with one ormultiple thiols to covalently bind to the Au nanoparticles. Thethiolated GOx is selectively deposited to the Au nanoparticles of thecarbon nanotube FET, using the above mentioned SAM passivation method.Fluorescent labels might also be used on the protein to confirm thesuccessful attachment using confocal fluorescent imaging techniques.After the device is fully functionalized, sensing tests can be taken.For glucose sensing, the device is placed in an aqueous environment. APBS buffer is applied to the nanotube device, and the device conductanceis monitored in real time with a constant gate. Then variousconcentrations of glucose solution are added into the buffer using apipette, and the change in conductance is recorded. The applied gatevoltage will be tested to find the most pronounced signal, and differentconcentrations will be added to measure the sensitivity of the device.Once nanotube device parameters are optimized for sensing glucose, theoptimized devices may be used as the sensor portion of the transdermaldevices described herein.

In an alternative embodiment to those described above, the CNTFET isdecorated with metallic (e.g., Au) nanoparticles that are pre-bound withselected bioreceptor molecules (e.g., thiolated GOx). The iSEconfiguration can have different shapes, as seen in FIG. 7, e.g., therectangular iSE could run parallel to the CNT, and shape of the iSE.Further still, bioreceptors could be pre-bound to the iSE prior toapplication of the external electric field, such that the metallicnanoparticles already include the bioreceptors at the time of decorationof the CNT. A masking layer is used over the electrodes, e.g.polymethylmethacrylate (PMMA) or the negative-tone photoresist known asSU-8 to protect the contact electrodes from decoration. The protectionlayer would form a physical barrier to the decorating species. Suchmaterials are described in J. Zhang, A. Boyd, A. Tselev, M. Paranjape,and P. Barbara, Mechanism of NO ₂ Detection in Carbon Nanotube FieldEffect Transistor Chemical Sensors, Appl. Phys. Lett. 88(12), 2006 whichis incorporated herein by reference in its entirety.

By way of non-limiting example, the decorated CNTFETs are used in placeof the photonic-based and other sensors described previously. Moreparticularly, referring, for example, to FIG. 2 of U.S. Pat. No.6,887,202, a decorated CNTFET would be in the position of detectionlayer 203 such that it is exposed to the sample from capillary 202.Using the Au decorated CNTFET with thiol-terminated glucose oxidasebioreceptors bound thereto, glucose present in the sample would reactwith the bioreceptors and cause a measurable electrical response.

Additional descriptive material that may be helpful to understanding thepresent embodiments is found in “Fabrication and Functionalization ofCarbon Nanotube Field Effect Transistors for Bio-Sensing Applications”by Jianyun Zhou, dated Dec. 17, 2009, submitted to the faculty of theGraduate School of Arts and Sciences of Georgetown University andincorporated herein by reference in its entirety.

It should be apparent to one of ordinary skill in the art that otherembodiments can be readily contemplated in view of the teachings of thepresent specification. Such other embodiments, while not specificallydisclosed nonetheless fall within the scope and spirit of the presentinvention. Thus, the present invention should not be construed as beinglimited to the specific embodiments described above, and is solelydefined by the following claims.

1. A process for forming a functionalized sensor for sensing a moleculeof interest comprising: providing at least one carbon nanotube having afirst and a second electrode in contact therewith on a substrate;providing a third electrode including a decorating material on thesubstrate a predetermined distance from the at least one carbon nanotubehaving a first and a second electrode in contact therewith, wherein thedecorating material has a bonding affinity for a bioreceptors that reactwith the molecule of interest; and applying a voltage to the thirdelectrode, causing the decorating material to form nanoparticles of thedecorating material on the at least one carbon nanotube.
 2. The processaccording to claim 1, wherein the third electrode is formed of a bilayerof Cr and Au.
 3. The process according to claim 2, wherein thedecorating material is Au.
 4. The process according to claim 1, whereinthe predetermined distance is approximately 5-20 μm from the first andsecond electrodes.
 5. The process according to claim 1, wherein a sizeand density of the nanoparticles is related to the applied voltage. 6.The process according to claim 1, further comprising passivating thesubstrate to reduce adsorption of the bioreceptors.
 7. The processaccording to claim 6, wherein passivating includes introducing a selfassembled monolayer (SAM) to the substrate, wherein the SAM includeshead group with an affinity for the substrate and a tail having proteinresistivity.
 8. The process of claim 7, wherein the substrate is SiO₂and the SAM is methoxy-terminated PEG
 2000. 9. The process of claim 1,further comprising binding the bioreceptors to the nanoparticles. 10.The process of claim 1, wherein the bioreceptors are thiol-terminatedglucose oxidase and the molecule of interest is glucose.
 11. The processof claim 1, wherein the at least one carbon nanotube is selected fromthe group consisting of single-wall carbon nanotubes and multi-wallcarbon nanotubes.
 12. The process of claim 2, wherein the Cr isapproximately 25 Å and the Au is approximately 800 Å and isapproximately 5 μm wide.
 13. The process according to claim 1, whereinthe third electrode is formed of a bilayer of Ti and Au.
 14. The processaccording to claim 1, wherein the at least one carbon nanotube is anetwork of carbon nanotubes.
 15. The process according to claim 14,wherein the network of carbon nanotubes includes carbon nanotubesselected from the group consisting of single-wall carbon nanotubes andmulti-wall carbon nanotubes.
 16. A system for forming a functionalizedsensor for sensing a molecule of interest comprising: at least onecarbon nanotube having a first and a second electrode in contacttherewith on a substrate; a third electrode including a decoratingmaterial on the substrate a predetermined distance from the at least onecarbon nanotube having a first and a second electrode in contacttherewith, wherein the decorating material has a bonding affinity for abioreceptors that react with the molecule of interest; wherein a voltageis applied to the third electrode, causing the decorating material toform nanoparticles of the decorating material on the at least one carbonnanotube.
 17. The system of claim 16, wherein the at least one carbonnanotube is selected from the group consisting of single-wall carbonnanotubes and multi-wall carbon nanotubes.
 18. The system according toclaim 16, wherein the third electrode is formed of a bilayer of Cr andAu.
 19. The system of claim 18, wherein the Cr is approximately 25 Å andthe Au is approximately 800 Å and is approximately 5 μm wide.
 20. Theprocess according to claim 16, wherein the third electrode is formed ofa bilayer of Ti and Au.
 21. The system according to claim 16, whereinthe decorating material is Au.
 22. The system according to claim 16,wherein the predetermined distance is approximately 5-20 μm from thefirst and second electrodes.
 23. The system according to claim 16,wherein a size and density of the nanoparticles is related to theapplied voltage.
 24. The system according to claim 16, wherein thesubstrate is passivated to reduce adsorption of the bioreceptors. 25.The system according to claim 24, wherein the substrate includes anassembled monolayer (SAM) having a head group with an affinity for thesubstrate and a tail having protein resistivity.
 26. The system of claim25, wherein the substrate is SiO₂ and the SAM is methoxy-terminated PEG2000.
 27. The system of claim 16, wherein the bioreceptors arethiol-terminated glucose oxidase and the molecule of interest isglucose.
 28. The system according to claim 16, wherein the at least onecarbon nanotube is a network of carbon nanotubes.
 29. The systemaccording to claim 28, wherein the network of carbon nanotubes includescarbon nanotubes selected from the group consisting of single-wallcarbon nanotubes and multi-wall carbon nanotubes.